The present invention relates to an on-board navigation system for an aircraft, such as an airplane, a missile or a craft.
Navigation of aircraft, missiles or spacecraft is generally afforded by employing one or more inertial navigation facilities, possibly combined with an altimeter measurement. The errors of calibration and of implementation associated with such inertial equipment inevitably give rise to permanent impairment of the accuracy of navigation during the flight of such craft.
When this navigation error grows too large, it becomes necessary to make an adjustment to the inertial information via a complementary means.
Numerous adjustment sensing means may be envisaged, such as for example a GPS (Global Positioning System) system for location by satellite, TERCOM adjustment by altimeter correlation, stellar observation, visible or invisible imaging, etc., each being defined by conditions of implementation and associated limits of use. With the advent of on-board high resolution radars, accurate adjustment of navigation by radar imaging is becoming conceivable.
This is particularly the case when the craft is equipped with a synthetic aperture sideways looking radar, known as SAR radar, which is an autonomous system which can be used in any weather and at any time and which makes it possible to envisage very accurate and very full adjustment of navigation.
There has already been proposed to this end an on-board navigation system for an aerial craft, of the type which includes an inertial navigation facility that delivers an estimation of the position and velocity of the vehicle with respect to the terrestrial surface, an SAR synthetic aperture sideways looking radar that produces an SAR radar image of the territory in the vicinity of the point overflown by the vehicle, memory means that contains a reference map of the territory overflown, means for comparison that aligns an SAR radar image produced in flight with the stored reference map and provides an alignment deviation in two perpendicular directions and computing means linked to the memory means, to the inertial navigation facility and to the means of comparison, which include filter means of the KALMAN type, and which deliver an inceptor to the KALMAN filter for the estimation of the inertial errors in position and in velocity as a function of the alignment deviation .delta..
Such a system is described and represented in the document James E. Bevington-Charles A. Martilla (Honeywell Systems and Research Center)-Precision Aided Inertial Navigation Using SAR and Digital Map Data IEEE PLAN' 1990: "Position Location and Navigation Symposium Record", March 1990, pages 490-496.
These papers are based on modeling in which the alignment deviation .delta. is merged directly with the inertial error in the horizontal plane containing the two perpendicular directions or require the use of an altitude reference (altimeter).
This modeling does not make it possible to get back directly to the inertial errors in altitude and/or in velocity and, if these inertial errors in velocity or in altitude are too great, adjustment of the horizontal position is corrupted by substantial errors.